1. Field of the Invention
The present invention relates to a thin film magnetic merged write head which has a well defined pole tip structure.
2. Discussion of the Related Art
In a magnetic disk drive, data is written and read by magnetic transducers called "heads" which are positioned over a disk while it is rotated at a high speed. Magnetic heads are supported over a surface of the disk by a thin cushion of air (an "air bearing") produced by the disk's high rotational speed. In order to increase the amount of data stored per unit of disk surface area ("areal density"), more data must be written in narrower tracks on the disk surface. Accordingly, areal density of magnetic recording can be improved by increasing the number of data tracks which a write head can record on a disk; the related parametric expression is "tracks per inch" or "TPI". The TPI capability of a write head is increased by decreasing the head dimension which determines the width of a data track; typically this dimension is called the head "track width".
In the magnetic recording technology, thin film magnetic heads are desirable because of their high resolution and high areal density respectively. They are also easy to manufacture. With various thin film manufacturing techniques, they can be fabricated in batches on a ceramic substrate and then cut into individual heads.
A thin film write head includes a pair of pole pieces that are formed from thin films ("layers") of magnetic material. These layers are called "pole layers". The pole layers have a pole tip height dimension commonly called "throat height". In a finished write head, throat height is measured between an air bearing surface ("ABS"), formed by lapping and polishing the tips of the pole layers ("pole tips"), and a zero throat height level ("zero throat level") where the first pole layer (P1) and the second pole layer (P2) converge at the magnetic recording gap.
A thin film magnetic write head includes a pole tip region which is located between the ABS and the zero throat level and a back area which extends back from the zero throat level to and including a back gap. The write head includes a yoke which has top and bottom pole layers, each pole layer having a pole tip portion in the pole tip region and a back portion in the back region. The pole layers are connected together at the back gap.
An important manufacturing objective is to precisely define the pole tip of the write head, thereby to maximize areal density. As is known, areal density is determined in part by the number of flux reversals per millimeter of track length, which, in turn, depends upon the length of the gap between the pole tips ("gap length"). By decreasing the gap length, the bit density within a track is increased. The shortness of the gap length is limited by the decreasing flux intensity between the pole tips. The lower limit of the gap length to optimize bit density and flux intensity is substantially 0.2 microns. Accordingly, efforts have not been made to reduce the gap length below 0.2 microns. Instead, efforts have been directed toward reducing the track width of the write head, which is determined by the width of the thin film layers forming the pole tips at the ABS.
The pole tips are extensions of the bottom and top pole layers (P1 and P2, respectively) of the write head. The bottom pole layer transitions to a pole tip in the pole tip region and the top pole layer transitions to a pole tip in the pole tip region. The pole tips are separated by a gap (G) which is a thin layer of insulation material. The pole tip of the top layer P2 is the last element to induce flux into a magnetic medium; therefore, its width is more important than the width of the pole tip on the bottom pole layer P1. However, it is desirable for the pole tips to have the same width so as to minimize flux leakage therebetween.
In the prior art it has been difficult to fabricate the pole layers with tips less than 2 microns in width. This is because the width of the top pole layer at the pole tip region is typically defined after placement of the coil structure, one or more insulation layers and the top pole layer. In the prior art the width of the pole tip of the top pole layer P2 is defined by forming a thick photoresist mask on top of the top pole layer and then etching the width configuration at the pole tip region. The thickness of the resist layer has to be 20 microns or more in order to protect the top pole layer from ion bombardment. When the resist layer is thick, it cannot be sharply configured by a photoresist mask. This in turn means that the subsequent step of etching cannot produce a pole tip with a width less than 2 microns.
Another problem in fabrication of thin film write heads is the difficulty in precisely positioning the zero throat level. The zero throat level is where the pole tip region of the write head transitions to the back region of the write head. The back region includes a coil structure sandwiched between insulation layers which, in turn, are sandwiched between the top and bottom pole layers. Because of the difficulty in maintaining well defined frontal configurations of the top insulation layer and the top pole layer at the zero throat level, the zero throat level is not well defined. It is desirable to provide the zero throat level in a well defined plane which is parallel to the plane of the ABS. The pole tips and the gap layer should have back surfaces which lie in a common plane which is parallel to the ABS. This will prevent flux leakage from the top pole layer into the back portions of the pole tip.
Still another problem in the prior art has been planarization of the write head structure during fabrication. During fabrication, the insulation layers which sandwich the coil structure flow into the pole tip region. This results in a high topography in the back region before the top pole layer is deposited. Planarization is important for the top pole layer so as to control its thickness and effectiveness.